Is Zizania aquatica hardy in the UK?

Zizania aquatica is rated RHS H5 (as overwintering seed; foliage is a single-season annual) (hardy in a cold winter, hardy to about -15 to -10°C). It hardy through most of the UK even in severe winters.

In most of the UK, yes, if your garden's coldest night stays within its RHS H5 (as overwintering seed; foliage is a single-season annual) band (around -15 to -10°C). In colder inland or northern gardens, give it a sheltered spot, sharp drainage, and fleece on hard-frost nights.

RHS hardiness ratings run H1a (needs a heated glasshouse) to H7 (survives below -20°C). Zizania aquatica's rating, H5 (as overwintering seed; foliage is a single-season annual), means hardy in a cold winter — it tolerates winter minimums of about -15 to -10°C. The rating is an absolute minimum, not an average.

For a borderline-hardy plant, the UK winter killers are not just cold air but cold wet roots and wind. Improve drainage before winter, mulch the root zone, move containers to a sheltered spot or against a warm wall, and use horticultural fleece on hard-frost nights. Established plants in free-draining soil take far more cold than a recently planted one in heavy, wet ground.

UK winters vary far more by locality than by latitude. Mild western and coastal areas (the south-west, Pembrokeshire, western Scotland's coast) and urban heat islands run several degrees warmer than inland frost pockets, valleys, and exposed eastern sites at the same latitude. A plant marginal on its RHS rating often survives in a sheltered town garden but fails 20 miles inland — so judge by your own coldest recorded night and microclimate (a south-facing wall, a courtyard) rather than the regional average.